Yellow banding on LRGB image - help!!

Hi Michael,

How did you register and combine the channels? Did you do this in Affinity Photo?
I also worked some time with Affinity to combine LRGB data and turned away from it as it handles the color combination IMHO not in the way we would like it. If you follow the Affinity tutorial instructions to recolor a layer to assign it to a channel, you'd be surprised. Also Luminance isn't Luminance. 
I'd point at this process. I could go into more detail but I don't know if you had used Affinity.

For removing the gradient and vignetting, you can use the "calibrate background filter" in Affintiy. It's part since version 1.9.0.

Cheers,
Björn

It's certainly not an alignment problem. There are a few things to consider:
1. If you use a recolor adjustment, you'd be surprised that red isn't red: Take a look at the layers for the different colors and look at the color channels. You'll see that the recolored red channel may still have some signal in the green and blue channel. It's how Affinity computes colors. Typical color combination processes assign the intensity purely to the according channel. Hence I recommend following test: don't use the recolor adjustment but simply delete the content of the blue and green channel of the red filter and for the other filters accordingly. Check if the yellow cast is still there. Don't use the luminance layer yet.
You can compensate this by setting the luminance to 50% in the recolor adjustment. Then all the information of the red filter will go into the red channel. Nevertheless, it's a lot of workarounds.

2. The luminance layer, actually the luminance blend mode, is luma and not luminance. It's weighted adjustment of the color channels. There is a thread in the Affinity forum regarding this. This is a very technical aspect: https://forum.affinity.serif.com/index.php?/topic/44069-luminosity-blend-mode-help/
This may introduce also a subtle color cast.

For these reasons, I turned away from Affinity to combine mono images and use dedicated software (AstroPixelProcessor) in my case. I use Affinity only with the color combined and already stretched image for minor contrast or brightness adjustments and annotations.

I hope I could explain this well. Please ask if something isn't clear.

Björn

I tried reprocessing the RGB differently avoiding recolour filters, instead extracting the individual channels of each RGB image as spare channels then assigning those spare channels to a new RGB pixel layer. Unfortunately same result with no improvement so must be the data somehow.

I'm going to notch this one down as a loss but will still post the image as broadly i'm happy with it albeit with the background banding, I'll keep an eye out if it appears again in future images to try and understand its cause. One to look back on in the future to know i've improved (hopefully!)

thanks for the help.
Michael

Hi Björn - yes it's only visible in the combined RGB image, the luminance layer quality is much better with no gradients at all.

I stretch the RGB image and the blue/yellow gradient appears then it only goes into a banding artefact once I create a synthetic flat frame to remove the gradient and subtract it from the RGB image. I'm thinking it's got to be something to do with that synethic flat and extracing it that is causing the banding. I do have to swap between 16 bit and 32 bit colour spaces to do it, maybe that is having an adverse impact?

FYI here's the final image i ended up with in case you're interested to see it > https://www.astrobin.com/3l383d/0/

Sound guidance again thank you, I'll try without subtracting the synthetic flat and see how it comes out. I've tried the background extraction tool but i feel it's quite over powering, maybe i just need to play about with the settings. 

The only filters that arent available in 32bit is median blur and/or dust and scratches which is needed for the synthetic flat frame BUT if i dont use one, no need to come out of 32 bit.

I always shoot flats and dark flats after each imaging run because i have a mobile setup so the orientation and focus is always slightly different (no matter how hard i try to keep it the same).

When you refer to filling layers, what exactly do you mean? I've not heard of that before but keen to learn. 

Thanks
Michael

Ah yes just a terminology thing i think, i essentially creatr a starless gradient layer to remove a background gradient in any image.

Filling later looks promising, i assume you create a gradient that matches the gradient on the image in terms of position/direction/colour then blend into the image to "paint over" the gradient? How do you do blend it considering there are stars in your working layer but won't be in the filler layer?

Thanks!